Problem

The Apache Struts framework uses I18NInterceptor to allow users and developers switch language used in the framework and an application built on top of it. The problem is that the interceptor doesn't perform any validation of the user input and accept arbitrary string which can be used by a developer to display language selected by the user. However, the framework doesn't expose the value directly in UI.

Solution

If you want present language selected by user based on I18NInterceptor always escape the string before presenting it to the user. Alternatively upgrade to Struts 2.3.28.

Backward compatibility

No issues expected when upgrading to Struts 2.3.28.

Workaround

When needed you can use StringEscapeUtils from the Apache Commons to escape the string.